FIG. 8 is a block diagram showing a schematic structure of a conventional general quadrature modulator.
Namely, in this quadrature modulator, a carrier signal c input from the exterior is directly applied as a new carrier signal cIN1 to a quadrature modulator 1, and the carrier signal which is phase-shifted by 90° at a 90 degree phase-shifter 2 is applied as a new carrier signal cIN2 to the quadrature modulator 1.
Further, the respective carrier signals cIN1 and cIN2 applied to the quadrature modulator 1 whose phases are perpendicular to one another, are respectively applied to multipliers 3 and 4.
An I signal (in phase component) and a Q (quadrature component) signal are respectively applied to the multipliers 3 and 4.
Multiplier 3 multiplies the carrier signal cIN1 by the I signal, and outputs a multiplied signal d1 to the adder 5.
Multiplier 4 multiplies the carrier signal cIN2 by the Q signal, and outputs a multiplied signal d2 to the adder 5.
The adder 5 adds the multiplied signals d1 and d2 respectively output from the multipliers 3 and 4, and outputs a modulating signal (a quadrature modulating signal) a.
In such a quadrature modulator 1, it is supposed that a case arises that the phases of a pair of carrier signal components c1 and c2 included in the modulating signal a output from the quadrature modulator 1 are not precisely perpendicular to one another due to, for example, a setting defect or an error of the 90 degree phase-shifter 2, and an error in the propagation delay time from the multipliers 3 and 4 to the adder 5.
In such a case, when the modulating signal a obtained by applying the I signal and the Q signal to the quadrature modulator 1 is demodulated into two signals by another quadrature modulator, a situation arises that the two demodulated signals are not completely separated into an I signal component and a Q signal component, and one signal component (information) leaks to the demodulated other signal component (information).
Accordingly, when the quadrature modulator 1 is built into various communication systems, it is necessary to measure a quadrature degree of the phases between the pair of carrier signal components c1 and c2 included in the output modulating signal a, and to suppress a quadrature error θ to less than or equal to a given tolerance limit.
In order to measure a quadrature error θ based on the phase difference between the pair of carrier signal components c1 and c2 input in the quadrature modulator 1, in a state that complex sine-wave signals serving as a reference are applied as the I signal and the Q signal, image (waveform) information of the modulating signal a output from the quadrature modulator 1 is analyzed by a measuring device such as a spectrum analyzer or the like.
Further, while the image (waveform) information of the modulating signal a is being monitored by the measuring device such as a spectrum analyzer or the like, separately, a phase difference between the carrier signal components c1 and c2 is adjusted so as to be 90° by a phase adjuster.
However, in the method described above, measuring devices such as a precision complex sine-wave signal generator, a high-priced spectrum analyzer, or the like are specially required.
However, it is practically impossible that such a complex and high-priced measuring device is built into a quadrature modulating apparatus.
Moreover, the adjusting operation that the phase difference between the carrier signal components c1 and c2 are adjusted by the use of the complex sine-wave signal generator and the spectrum analyzer is complex, and executed by the intuition and the experience of a skilled operator. However, individual differences among adjustment operators greatly affect results of adjustment, and there is the concern that the adjusted results greatly fluctuate.
Further, there is the problem that an operator who is inexperienced in the adjustment cannot execute the adjusting operation.